On a conjecture concerning dot-depth two languages

AbstractIn this paper, we study the second level of the dot-depth hierarchy for star-free regular languages. We investigate a necessary condition stated by Straubing for a language to have dot-depth two, and prove that it is sufficient for languages whose syntactic monoid is inverse with three inverse generators. Also we disprove a conjecture according to which Straubing's condition would be equivalent to both dot-depth two and another condition expressed in terms of two-sided semidirect product

Mg II Absorption Systems in SDSS QSO Spectra

We present the results of a MgII absorption-line survey using QSO spectra from the SDSS EDR. Over 1,300 doublets with rest equivalent widths greater than 0.3\AA and redshifts $0.366 \le z \le 2.269$ were identified and measured. We find that the $\lambda2796$ rest equivalent width ($W_0^{\lambda2796}$) distribution is described very well by an exponential function $\partial N/\partial W_0^{\lambda2796} = \frac{N^*}{W^*} e^{-\frac{W_0}{W^*}}$, with $N^*=1.187\pm0.052$ and $W^*=0.702\pm0.017$\AA. Previously reported power law fits drastically over-predict the number of strong lines. Extrapolating our exponential fit under-predicts the number of $W_0 \le 0.3$\AA systems, indicating a transition in $dN/dW_0$ near $W_0 \simeq 0.3$\AA. A combination of two exponentials reproduces the observed distribution well, suggesting that MgII absorbers are the superposition of at least two physically distinct populations of absorbing clouds. We also derive a new redshift parameterization for the number density of $W_0^{\lambda2796} \ge 0.3$\AA lines: $N^*=1.001\pm0.132(1+z)^{0.226\pm0.170}$ and $W^*=0.443\pm0.032(1+z)^{0.634\pm 0.097}$\AA. We find that the distribution steepens with decreasing redshift, with $W^*$ decreasing from $0.80\pm0.04$\AA at $z=1.6$ to $0.59\pm0.02$\AA at $z=0.7$. The incidence of moderately strong MgII $\lambda2796$ lines does not show evidence for evolution with redshift. However, lines stronger than $\approx 2$\AA show a decrease relative to the no-evolution prediction with decreasing redshift for $z \lesssim 1$. The evolution is stronger for increasingly stronger lines. Since $W_0$ in saturated absorption lines is an indicator of the velocity spread of the absorbing clouds, we interpret this as an evolution in the kinematic properties of galaxies from moderate to low z.Comment: 50 pages, 26 figures, accepted for publication in Ap

Study of the local field distribution on a single-molecule magnet-by a single paramagnetic crystal; a DPPH crystal on the surface of an Mn12-acetate crystal

The local magnetic field distribution on the subsurface of a single-molecule magnet crystal, SMM, above blocking temperature (T >> Tb) detected for a very short time interval (~ 10-10 s), has been investigated. Electron Paramagnetic Resonance (EPR) spectroscopy using a local paramagnetic probe was employed as a simple alternative detection method. An SMM crystal of [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-acetate) and a crystal of 2,2- diphenyl-1-picrylhydrazyl (DPPH) as the paramagnetic probe were chosen for this study. The EPR spectra of DPPH deposited on Mn12-acetate show additional broadening and shifting in the magnetic field in comparison to the spectra of the DPPH in the absence of the SMM crystal. The additional broadening of the DPPH linewidth was considered in terms of the two dominant electron spin interactions (dipolar and exchange) and the local magnetic field distribution on the crystal surface. The temperature dependence of the linewidth of the Gaussian distribution of local fields at the SMM surface was extrapolated for the low temperature interval (70-5 K)

Star Formation, Supernovae Feedback and the Angular Momentum Problem in Numerical CDM Cosmogony: Half Way There?

We present a smoothed particle hydrodynamic (SPH) simulation that reproduces a galaxy that is a moderate facsimile of those observed. The primary failing point of previous simulations of disk formation, namely excessive transport of angular momentum from gas to dark matter, is ameliorated by the inclusion of a supernova feedback algorithm that allows energy to persist in the model ISM for a period corresponding to the lifetime of stellar associations. The inclusion of feedback leads to a disk at a redshift $z=0.52$, with a specific angular momentum content within 10% of the value required to fit observations. An exponential fit to the disk baryon surface density gives a scale length within 17% of the theoretical value. Runs without feedback, with or without star formation, exhibit the drastic angular momentum transport observed elsewhere.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter

Electrically detected magnetic resonance of carbon dangling bonds at the Si-face 4H-SiC/SiO2_2 interface

SiC based metal-oxide-semiconductor field-effect transistors (MOSFETs) have gained a significant importance in power electronics applications. However, electrically active defects at the SiC/SiO$_2$ interface degrade the ideal behavior of the devices. The relevant microscopic defects can be identified by electron paramagnetic resonance (EPR) or electrically detected magnetic resonance (EDMR). This helps to decide which changes to the fabrication process will likely lead to further increases of device performance and reliability. EDMR measurements have shown very similar dominant hyperfine (HF) spectra in differently processed MOSFETs although some discrepancies were observed in the measured $g$-factors. Here, the HF spectra measured of different SiC MOSFETs are compared and it is argued that the same dominant defect is present in all devices. A comparison of the data with simulated spectra of the C dangling bond (P$_\textrm{bC}$) center and the silicon vacancy (V$_\textrm{Si}$) demonstrates that the P$_\textrm{bC}$ center is a more suitable candidate to explain the observed HF spectra.Comment: Accepted for publication in the Journal of Applied Physic